In the production of recycled paper from relatively high quality stocks, major difficulty in producing a high quality end product is the removal of sticky contaminants from the pulp. "Sticky contaminants" as that term is used in the art relates to a wide variety of tacky contaminants including tapes, labels, and hot melt glues. The major difficulty with sticky contaminants is that they can bleed or otherwise come out of the pulp at any time, including when the pulp stock first hits the wire of the paper making machine, or later on during converting or printing stages. Sticky contaminants have a major adverse impact on runnability of the final paper produced.
Primary sources of sticky contaminants include single side pressure sensitive tapes of all types, double face splicing tapes, hot melt glue such as used in bookbindings, pressure sensitive labels, self-seal envelopes, and stick-on notes. With the exception of bookbinding hot melts (which may be removed by screening and cleaning), all are extraordinarily difficult to remove by conventional technology. As a matter of fact, normal deinking formulations, if anything, appear to enhance the tackiness of the pressure sensitive adhesives and thereby contribute to the problem.
As used in the present specification and claims, "sticky contaminants" is intended to refer to all types of sticky contaminants from the sources as set forth above, except for bookbinding hot melts.
Many of the sticky contaminants are plastics, and the vast majority are of relatively lightweight material, having a specific gravity less than that of the pulp which they are contaminate. Those that are plastics by their very nature are hydrophobic, and it is possible to render other sticky contaminants hydrophobic too. According to the present invention, the relatively low specific gravity and the hydrophobic nature of the sticky contaminants are taken advantage of in effecting removal. The removal according to the invention can be practiced independent of deinking--that is, for example, in a stage prior to deinking in which no deinking facilitating chemicals have yet been added--or it may take place simultaneously with deinking.
According to one aspect of the present invention, a method of removing sticky contaminants from recycled paper cellulosic pulp containing such contaminants is provided by practicing the steps of: continuously reducing paper having sticky contaminants associated therewith to a finely comminuted liquids-solids suspension flow having a low solids consistency; introducing the suspension into a first end of a vortex; sparging air into the suspension in the vortex to cause sticky contaminants to attach to air bubbles and move toward the first end of the vortex, a foam being formed, while the pulp moves to the second end, opposite the first end, of the vortex; and removing foam and lightweight sticky contaminants from the first end of the vortex while pulp without (or with less) sticky contaminants is discharged from the second end of the vortex. The lightweight component of the sticky contaminants tends to migrate radially toward the center of the vortex, and thus ultimately is rejected with the foam of hydrophobic particles and air bubbles. Also, since the sparging air is reduced radially inwardly into the suspension, the radial inward movement of the lightweight material of the sticky contaminants toward the center of the vortex is enhanced, providing another motive force contributing to removal of sticky contaminants from the pulp. The hydrophobic sticky contaminants, such as plastics, attach to air bubbles being introduced into the vortex, foaming, and being removed as foam from the first end of the vortex.
The method according to the invention may be practiced on suspensions having a solids consistency of about 1-4% (e.g., over 1.5%), and the pulp may be deaerated after discharge from the vortex. Further, the gas component of the foam removed from the vortex first end may be separated from the solids and liquid component thereof by feeding the foam to a cyclone. While the hydrocyclone used to practice the invention may have a variety of different orientations, it preferably has a vertical axis with the first end at the top and the second end at the bottom, and the sparging air is directed through porous side walls of the hydrocyclone to create a foam of fine bubbles.
If necessary, the pulp flow may be broken up into a plurality of different flow paths and the sticky contaminant removing steps (sparging of air into the vortex, etc.) practiced in each of the flow paths, with the flow path ultimately recombined. A number of in-series stages may be provided within each flow path for treatment to remove sticky contaminants, and the pulp may be deaerated between each stage within each flow path. Again, sticky contaminants removal may be practiced prior to or simultaneously with deinking utilizing the same equipment.
According to another aspect of the present invention a method of manufacturing recycled paper is provided. Cellulosic paper having sticky contaminants associated therewith is reduced to a finely comminuted liquid solids suspension flow, and is screened, and then subjected to froth flotation. The particular froth flotation process is as described above, i.e. utilizing a vortex and air sparging. The pulp produced is washed and thickened, and paper is made from the thickened pulp.
The invention also relates to a recycled paper pulp substantially free of sticky contaminants which is produced from the contaminated pulp described above. While conventional recycled pulps free of sticky contaminants can typically only be produced by careful source separation to remove as many of the possible contaminating sources as possible, according to the invention the sticky contaminants can actually be removed, thereby providing a solution to the paper recycling industry's most universal and serious problem.
It is a primary object of the present invention to provide for the effective removal of sticky contaminants from recycled paper cellulosic pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.